1. Field of the Invention
The present invention relates to semiconductor devices and, more particularly, to a semiconductor memory device that includes a reference cell to set a reference level for reading and writing independent of memory cells, and uses the reference level during data writing and data read verify.
2. Description of the Related Art
In a semiconductor device, especially in a non-volatile memory device, reference memory cells for reference at the time of memory cell reading are provided independently of regular memory cells. For instance, a core cell, a first internal reference cell, and a second internal reference cell are provided in a core cell region that stores data. A reference voltage is generated based on outputs of the first internal reference cell and the second internal reference cell. The data read out from the core cell is compared with the reference voltage to determine whether the read data is “0” or “1”.
The core cell, however, deteriorates as data is written and erased. A flash memory device stores information in accordance with the amount of charges accumulated in a charge storing layer thereof. As a result, a deterioration phenomenon is observed wherein charges fall off with time. Therefore, a reference cell is provided in the core cell region, so that the data deterioration characteristics of the core cell data match those of the reference cell data. After the data in the reference cell is erased together with the data in the core cell, the reference cell is reconfigured.
FIG. 1 shows the threshold voltage distribution of the first internal reference cell and the second internal reference cell. Since each of the first and second internal reference cells is formed with several cells, each threshold value exhibits a certain variation. As a result, a reference voltage Vref generated based on the average current of those cells also exhibits a certain variation. While the reference voltage Vref can vary with the threshold voltage due to core cell deterioration, the read margins (ΔMG0, ΔMG1) become smaller.
To counter this problem, Japanese Unexamined Patent Publication No. 2004-110881 discloses a technique for generating a reference voltage Vref based on the outputs of first and second internal reference cells 102 and 103, and the output of a further provided external reference cell 104 as shown in FIG. 2. Since data rewrite is not performed in the external reference cell 104, changes with age are not caused therein. Also, since the external reference cell 104 is formed with a single memory cell, the threshold value does not have a distribution width. Accordingly, the average of the outputs of the three reference cells is set as the reference voltage Vref, to narrow the threshold voltage distribution of a virtual reference cell. Thus, data can be read out with accuracy. Here, the “threshold voltage distribution of a virtual reference cell” collectively represents the voltage distributions of all the reference cells to be used for generating a reference voltage.
However, the amount of change with age varies among the reference cells, and the problem of a decrease in margin at the time of reading is still present, even with the addition of the external reference cell.
The reference voltage Vref is generated based on the output of the external reference cell that does not change charge storing characteristics with time, and the outputs of the first internal reference cell and the second internal reference cell that change charge storing characteristics with time. Due to the change with time caused by rewrites of the first internal reference cell and the second internal reference cell, the reference cell of data “0” approaches the voltage output from the external reference cell by ΔV1, and the reference cell of data “1” separates from the voltage output from the external reference cell by ΔV1. In that case, the distribution of a virtual reference cell deteriorates by ΔV3=(ΔV1+ΔV2)/3.
FIG. 3 shows a case where the variation ΔV2 is wider. In that case, the distribution of a virtual reference cell is not greatly affected by the voltage of the external reference cell. As a result, the distribution of the reference cell of data “0” approaches the distribution of a virtual reference cell, and the margin between them (ΔMG0 in FIG. 3) vanishes.